1. Field of the Invention
The present invention relates to a pyroelectric-type IR receiving element for receiving infrared (IR) rays radiated from an object, and an improved IR sensor using the same receiving element.
2. Disclosure of the Related Art
Pyroelectric-type IR receiving elements are used as sensors for detecting a moving object such as a human. For example, a pyroelectric-type IR sensor is shown in FIG. 26. The IR sensor comprises an IR receiving element 100 that is generally known as a dual-type receiving element, a circuit board 120 to which the receiving element is fixed, a base 130 having three output pins 131 projecting downwardly therefrom, and a package cover 140 having an IR transmittable window 141. As shown in FIGS. 27A and 27B, the receiving element 100 comprises a substrate 101 made of a pyroelectric material, a pair of first and second electrodes (102a, 103a) formed on a top surface of the substrate, and a pair of third and fourth electrodes (102b, 103b) formed on a bottom surface of the substrate, and a pair of positive and negative output terminals 104 and 105 formed on both of the top and bottom surfaces at opposite ends of the substrate. The substrate 101 is made of a ceramic material such as PBTiO.sub.3 or PZT, a single crystal such as LiTaO.sub.3, or a high molecular compound such as PVF.sub.2. The four electrodes are made of an IR absorbent material such as NiCr or the like, and have the same rectangular shape. The first and second electrodes (102a, 103a) are arranged such that a longitudinal side of the first electrode extends parallel to that of the second electrode. The third and fourth electrodes (102b, 103b) are arranged so as to be respectively overlapped with the first and second electrodes (102a, 103a) through the substrate 101. The first and fourth electrodes (102a, 103b) are connected to the positive output terminal 104 by conductive patterns 106. Similarly, the second and third electrodes (103a, 102b) are connected to the negative output terminal 105 by conductive patterns 107. As a result, a pair of IR receiving portions 102 and 103 are defined on the pyroelectric substrate 101. When the IR receiving portions 102 and 103 receive IR rays, the received IR rays are converted to heat energy so that electric charges are generated. According to the electric charges, a voltage-difference signal is obtained from the IR receiving element 100 through a field-effect transistor (FET) 123 and a high resistance element 124, as shown in FIG. 27C. The receiving element 100 is fixed to a pair of stands 121 formed on the circuit board 120 in a bridge fashion. The circuit board 120 mounting thereon the receiving element 100 is attached to the base 130, and then covered by the package cover 140.
To obtain a higher IR sensitivity of the sensor, it is desired to improve light-heat exchange efficiency. For this purpose, a thermal-insulation property of the pyroelectric substrate 101 is improved, and a thin pyroelectric substrate having a thickness of 40 to 100 .mu.m has been used in the past. However, since the IR receiving element 100 having such a small thickness is fixed on the stands 121 in the bridge fashion by a conductive bond 150, as shown in FIG. 28A, thermal stress resulting from a difference of thermal expansion coefficients between the pyroelectric substrate 101 and the stand material 121 or the conductive bond 150 is applied to the pyroelectric substrate by a temperature change of the circumference even when no IR rays strike the IR receiving element 100, as shown by a two-way arrow of FIG. 28B. In general, the thermal stress tends to concentrate at defect portions, chipping portions, or micro-cracks in the pyroelectric substrate. This stress concentration brings about the occurrence of undesired electric charges. As a result, there is a problem in that unexpected noise signals generally known as a popcorn noise are output from the IR receiving element.
In the past, it has been proposed to select an adequate combination of the pyroelectric material and the stand material or the conductive bond to reduce the popcorn noise. However, the popcorn noise cannot be sufficiently reduced by only the material selection. Therefore, there is room for further improvement to reduce the popcorn noise.